1. Field of the Invention
The invention pertains to the manufacture of phosphorus pentoxide in general and in particular to recovering waste heat values during the process while preventing the corrosive effects of meta and polyphosphoric acid impurities formed during the process.
2. Related Art
The high heat of combustion of phosphorus to phosphorus pentoxide and ultimately to produce phosphoric acid equals about 10MBTU/lb. (730 Kcal/mol of P.sub.4), and can be utilized in the evaporation of water from dilute phosphate solutions or phosphoric acid solutions that require concentration before subsequent processing.
In the prior art manufacture of phosphorus pentoxide (P.sub.2 O.sub.5) and ultimately phosphoric acid, phosphorus is burned in excess air and the resulting P.sub.2 O.sub.5 is hydrated. Heat transfer surfaces utilized in the prior art as a means of recovering some of the heat of combustion are subject to severe corrosion as a result of the formation and build-up of metaphosphoric and polyphosphoric acids (hereinafter meta and polyacids) during heat recovery.
It has been suggested to recover low level (&lt;100.degree. C.) heat from wet manufacturing processes by operating on the Rankine cycle. This process, however, would only recover the low level heat which amounts to &lt;5% of the heat value realized.
Derwent Publication, No. 60687B (1979), reported on Japanese Pat. No. 84,890 which is said to disclose a phosphoric acid plant wherein corrosion is prohibited by predrying the combustion air to prevent the formation of corrosive polyphosphoric acids, and heat-transfer surfaces are operated above the dewpoint of P.sub.2 O.sub.5. However, during process upsets and shutdown, the presence of moisture can still cause severe corrosion problems.
An article in Chemical Week, Jan. 31, 1979 issue, page 36 disclosed that Battelle Laboratories was utilizing Battelle's multisolid fluidized-bed combustion process to burn solids and sewage sludges. The unit is described as burning waste mixed with sand while the mixture is suspended in a vertical air stream. The hot sand is then separated and recirculated through an external boiler where it is utilized to dry the sludge and vaporize water to produce steam.
It is an an object of the invention to provide a process for the manufacture of phosphorus pentoxide which can be further treated to produce phosphoric acid whereby the waste heat, generated in the manufacture of the P.sub.2 O.sub.5 intermediate, can be recovered.
The invention discloses a process for recovering a major portion of the heat values generated in the manufacture of phosphorus pentoxide at high temperature levels while eliminating corrosion problems associated with prior art processes.